The present invention relates in general to an antenna for electromagnetic radiation, and in particular, but not exclusively, to an antenna capable of radiating broadband signals without significantly varying the relative phase of the frequency components in the spectrum of the radiated signal.
Most known antennas are inherently xe2x80x98narrow bandxe2x80x99; in other words they only radiate efficiently over a narrow range of radio frequencies. A significant minority are of xe2x80x98widebandxe2x80x99 design, but are almost invariably intended for reception of a plurality of narrow band signals spread over a wide range of frequencies. For example, the narrow band signals would normally be independent information channels spread over such a range. The relative phase of the frequency components is thus of no importance and the known wideband antennas tend to impose wide and uncontrolled phase variations on the frequency components.
A widely known example of the prior art is the log-periodic antenna, which comprises a series of elements that resonate at discrete frequencies across the operating band. The feed point to such an antenna is applied at the high frequency end. Waves travel down the feed line until they encounter an element which is approximately resonant and are radiated by that element. A longer clement behind the resonant element acts as a reflector and a shorter element in front as a director, thus causing radiation preferentially in the direction back towards the feed point. It has been found that the result of this mode of operation is that there is a time delay that increases with reducing frequency, imposed by the time taken to travel down the transmission line to the radiating element and then back again in the forward radiating direction. This time delay translates into a frequency-dependent phase variation (dispersion).
A second known design is the horn antenna, which can be of the plain pyramidal type or the wider-band ridged-guide type The typical horn antenna comprises a gently-tapered waveguide which progressively launches waves into space and is inherently of a broadband nature. However, there is still a degree of frequency-dependent phase dispersion in the waveguide portion and horn antennas are also very bulky relative to their operating frequency range.
A third known design is the planar spiral antenna commonly used for radar warning receivers. The planar spiral antenna is inherently circularly polarized and has a linearly-polarised relative, the sinuous antenna. By being flat, these designs inherently have a poorer directivity than the log-periodic or horn types, and in particular they radiate equally backwards as well as forwards. The result is that a layer of absorbent material is required behind the back face which inherently absorb 50% of the radiation. Even in the forward direction the beam shape is very wide. Therefore, radiation in any particular direction is relatively inefficient.
All of the prior art antennas mentioned above suffer to some extent from the same phase dispersion problems as described above with reference to the log-periodic antennas.
An antenna that is known to be inherently broadband and to have very little phase dispersion is the resistively-loaded dipole and several designs for this exist. However such dipoles inherently have a high degree of loss. Further, they produce an omnidirectional radiation pattern.
It is desired to provide an antenna for radiating broadband signals without significantly varying the relative phase of the frequency components in the spectrum of the signal. Preferably, the desired antenna should be compact and inexpensive to manufacture, and show directivity and high efficiency.
According to the present invention there is provided a broadband antenna, comprising: input means for receiving an input signal; a plurality of antenna elements for radiating said input signal; and signal feed means for feeding said input signal from said input means to said plurality of antenna elements; characterised in that: said signal feed means comprises a plurality of independent feed lines each coupled to a respective antenna element of said plurality of antenna elements.
Preferably, each of said plurality of feed lines comprises a delay for delaying the input signal by a predetermined delay amount.
Preferably, said predetermined delay amount is determined by the length of each respective feed line of said plurality of feed lines.
Preferably, said predetermined delay amount compensates for a phase delay between a position of the respective antenna element of said plurality of antenna elements and a predetermined phase reference point.
Preferably, said plurality of antenna elements are arranged to form a log-periodic antenna and said predetermined reference point is at the front of the antenna.
Preferably, said plurality of feed lines are provided by a circuit boards
Preferably, said plurality of antenna elements are arranged generally parallel and co-planar, and said circuit board is arranged normal to said antenna elements
Preferably, said signal feed means comprises a feed network provided on said circuit board for splitting, delaying and feeding at least a respective potion of the input signal to each respective antenna element.
Preferably, said feed network comprises a common transmission line coupled to receive said input signal and coupled to each of said plurality of feed lines.
Preferably, said common transmission line is tapered to act as an unbalanced to balanced transformer.
Preferably, said feed lines are arranged to reverse the phase of each alternate antenna element of said plurality of antenna elements.
According to a second aspect of the present invention there is provided a broadband antenna, comprising: a plurality of antenna elements arranged to provide a log-periodic array of dipole elements for radiating an input signal supplied by a signal feed means, characterised in that said signal feed means comprises a plurality of feed lines each coupled to a respective antenna element of said plurality of antenna elements, each feed line being of a predetermined length such that all frequency components of the radiated signal across a predetermined operating range are in phase at a predetermined phase reference point.
Preferably, each feed line comprises a delay for delaying the input signal, or part of the input signal applied to the respective antenna element, by a pre-determined delay amount.
Each element of the antenna preferably radiates optimally at a selected frequency, with relatively long elements radiating at relatively low frequencies, and relatively short elements radiating at relatively high frequencies. Preferably, the length of each respective feed line is selected to compensate for the phase delay between the position of the radiating element and a phase reference point at the front of the antenna. Thus, by selectively delaying each frequency portion of the signal by adjusting the physical length of the delay lines, all frequency components of the radiated signal leave the phase reference point at the front of the antenna in phase.
In a preferred embodiment, the independent feed lines are formed by etching a feed network onto a printed circuit board. Preferably, the printed circuit board is arranged in the plane of a central longitudinal axis of the antenna, and normal to the plane of the individual antenna elements. Since the feed network is normal to the direction of the electric fields radiated by the antenna, distortion is minimised.
The preferred antenna comprises feed lines etched in a feed network on a printed circuit board, with the length of each feed line being, in principle, chosen to compensate for the phase delay between the position of the radiating element and the phase reference point at the front of the antenna. This approach may be contrasted with the use of an electronically pre-distorted input signal intended to compensate for time delays in the antenna by subjecting an input signal with in-phase components to varying amounts of delays for various frequency components of the signal, to compensate for the time delay between a common input point and a respective radiating element.
The antenna described herein is intended particularly for use in radiating high power pulses efficiently, such as for use in the electromagnetic compatibility testing of electronic equipment. The antenna is also intended to be used to receive pulsed signals from interference sources, such as electrostatic discharges, lightning, or simulated nuclear electromagnetic pulses. Further applications include wideband ground probing radar and very wideband communications where the dispersive nature of standard antennas would be unacceptable.